1. Field of the Invention
The present invention relates generally to a servo valve device in a power steering apparatus used on a vehicle, and more particularly to a servo valve device provided with two flap valve members for controlling a fluid pressure motor in the power steering apparatus.
2. Description of the Prior Art
A servo valve device of the above type which is in current use is composed of a main valve member which is disposed to rotate in response to the action of the fluid pressure motor supplementing manual steering torque and which is provided with first and second valve chambers respectively connected to fluid supply and exhaust lines, and first and second flap valve members respectively disposed in the first and second valve chambers for rotating in response to the manual steering torque to control fluid under pressure for the fluid pressure motor. The main valve member is also provided with a pair of distribution ports opened at opposing walls in the first valve chamber and respectively communicating with operative ports of the fluid pressure motor for supply of fluid under pressure, and with a pair of injection ports opened at opposing walls in the second valve chamber and respectively communicating with the operative ports of the fluid pressure motor for exhausting the fluid.
In such a servo valve device, the two valve chambers are respectively formed within the main valve member by machinings, such as millings or grindings, given separately from each other, to be axially spaced. For that reason, it is very difficult to finish the two valve chambers in ideal size, form and position relative to each other in such a manner that gaps between both sides of the first flap valve member and both side walls of the first valve chamber and gaps between both sides of the second flap valve member and both side walls of the second valve chamber have a predetermined ideal relation to each other under a normal condition. Accordingly, it is very difficult for such a servo valve device to achieve ideal fluid control.
Moreover, generally, such two valve chambers are respectively formed from opposite ends of the main valve member. For that purpose, nothing can be integrally formed with the opposite ends of the main valve member, for example, a first steering rod for connecting the main valve member to the fluid pressure motor cannot be integrally formed with the main valve member. Accordingly, it is very difficult to exactly align the main valve member with the first steering rod, and the component parts of the servo valve device are thus inevitably increased in number.